Plasma guns are utilized for such purposes as thermal spraying which involves the heat softening of a heat fusible material, such as a metal or ceramic, and propelling the softened material in particulate form against a surface to be coated. The heated particles strike the surface and bond thereto. The heat fusible material is typically supplied to the plasma spray gun in the form of powder that is generally below 100 mesh U.S. standard screen size to about 5 microns.
In typical plasma systems an electric arc is created between a water cooled nozzle (anode) and a centrally located cathode. An inert gas passes through the electric arc and is exited thereby to temperatures of up to 15,000 degrees Centigrade. The plasma of at least partially ionized gas issuing from the nozzle resembles an open oxy-acetylene flame. A typical plasma flame spray gun is described in U.S. Pat. No. 3,145,287.
Plasma guns generally are capable of operating on either argon or nitrogen as the primary plasma gas. For argon the gas is introduced into the chamber near the cathode through one or more orifices with a tangential component to cause a vortical flow to the plasma as shown and described, for example, in U.S. Pat. No. 3,823,302. The reason is, that without the vortex, the arc is not carried far enough down the nozzle, resulting in low voltage and low efficiency. On the other hand, radially directed gas input may be used, as shown in aforementioned U.S. Pat. No. 3,145,287. Radial input is generally selected for nitrogen because a vortex tends to extend the nitrogen arc a long distance down the bore of the nozzle causing difficulty in starting the arc.
However, without a vortex for nitrogen the voltage and efficiency are low. Therefore, a secondary gas such as hydrogen is added, having the effect of improving these factors. When argon is used, even with a vortex the efficiency is undesirably low. Hydrogen is again added where possible, but that gas is often considered undesirable as it may cause brittleness in the sprayed coating. Helium is an alternative secondary gas but is expensive and less effective.
Generally each plasma spray gun is set up for a particular type of plasma forming gas, either with radial or tangential inlet. Guns that may be used for either primary gas typically have different gas distribution rings inserted near the cathode, requiring disassembly to change gases. Several efforts have been made to simplify the change. U.S. Pat. No. 3,313,908 discloses a plasma torch with two types of gas inlet ports for different gases that are selected alternatively by means of either of two external gas conduit fittings. This method still requires changing fittings on the gun.
U.S. Pat. No. 3,851,140 shows a plasma spray gun with a gas distribution ring having primary openings slanted forwardly and secondary openings oriented tangentially. The two sets of inlet openings function concomitantly to provide a helical flow component circumscribing the linear component. This ring is said to controllingly alter the gas flow, but there is no means to alter the flow for different gases without changing rings, nor is there means to change the type of flow during operation.
In U.S. Pat. No. 2,941,063 there is depicted a plasma torch in which gas is introduced at two separate locations. Near the cathode a radial source inlet introduces gas through an orifice into a chamber proximate the cathode to provide gas flow for the initial portion of the arc and associated plasma. A tangential gas source is directed into a second annular chamber of large diameter that is well downstream of the radial inlet chamber. These widely separated gas inlet sources are directed to different portions of the arc and do not provide for gas inlet choice or control in the proximity of the cathode.
In view of the foregoing an object of the present invention is to provide a novel gas distribution ring for use in a plasma gun, which permits a simple selection of either radial or vortical gas flow in the arc region of the gun.
Another object is to provide a novel gas distribution ring for use in a plasma gun, which permits separate radial and tangential gas inlets and contemporaneous regulation thereof.
Yet another object is to provide a novel gas distribution ring incorporating a means for isolating gas inlet ducts having different inlet flow characteristics.
A further object is to provide an improved plasma gun containing a gas distribution ring that allows selection between radial and tangential gas inlet without alteration of the gun.
These and still further objects will become apparent from the following description read in conjunction with the drawings.